This invention relates to military time fuzes, and more particularly, to an improved, pyrotechnic delay system therefor.
Pyrotechnic delay fuzes presently in use consist of one or more pressed pyrotechnic delay rings to provide predetermined time intervals between various functioning phases of munition systems. Prior to the present invention, the functioning time for such pyrotechnic delay fuzes was provided by the burning of a fixed amount of black powder or other applicable delay composition which was pressed into a narrow channel within the delay ring. Both the total length of the channel and the intrinsic burning rate of the utilized delay composition predetermines the maximum time capabilities of the system.
The delay composition which is usually black powder is characterized by several deficiencies. First, its marked hygroscopicity with accompanying complex reactions with moisture necessitates special loading procedures to equilibrate powder conditioning. Secondly, its intrinsic burning rate limits the maximum time capabilities of pyrotechnic delay fuzes.
To provide systems with greater latitudes in delay time capabilities and reduced hygroscopicity problems with consequent improved reliability of performance, a need has existed over the past years to find a suitable chemical replacement for the black powder delay composition in fuze delay rings. Past attempts to solve this need involved the use of either a slow-burning black powder composition, designated "coal powder" or gasless pyrotechnic delay composition.
The slow-burning black powder composition has been found unsuitable due to its limited increase in maximum delay time capabilities and its decrease in system reliability as the result of nonuniformity of the supplied material.
The gasless pyrotechnic delay composition had been found unsuitable due to poor propagative characteristics in fuze delay rings. The propagative burning of a pressed column of this type of composition is a combustion reaction in which the fuel and oxidant react to give essentially solid products. Any gaseous products present are chiefly the result of traces of organic materials in the formulation, or impurities in the constituents. As the prime reaction products of the burning gasless pyrotechnic delay composition are continually forming a hard, semi-permeable expanding slag, the aforementioned gases become pressurized. This pressurization increases until relief can be obtained via exit from the delay system. The pressure relief occurs in the form of sporadic partial venting between the delay rings or between a delay ring and the contiguous fuze components. The sporadic venting, which could be very brisant, indicates that the pressure build-up along the delay's reaction interface is not uniform. Both the expanding slag and the erratic pressure are manifested by the poor propagative characteristics of the gasless pyrotechnic delay compositions in the fuze delay rings. First, the uncontrolled, expanding slag caused the separation and misalignment of the delay rings with consequent occurences of nonpropagation of the delay train. Secondly, as the burning of a material at its reaction interface is directly related to pressure, nonuniformity in the latter produced erratic propagation in the former.
The present invention entails the successful development of delay cord to be used as a replacement for conventional delay rings containing black powder and gasless pyrotechnic delay composition in time fuzes. This invention utilizes the pyrotechnic delay composition in the form of a cord placed in a spiral race of the fuze. The use of the pyrotechnic delay composition disposed in a spiral race, rather than in conventional delay rings, permits longer delay periods, greater burning time reliability by reduced susceptibility to changes in ambient pressure, etc., and accurate adjustment and control of delay periods.